Blades of turbomachines, in particular turbine blades, are exposed to very high material loading and correspondingly high wear during operation. For this reason, in the context of maintenance work, the blades are normally replaced with new blades after a predetermined operating time. The operating time is advantageously chosen such that the blades which have been removed can still be repaired.
Replacing the blades of a turbine involves replacing the turbine rotor blades, the turbine guide vanes and the guide segments. In order to be able to ensure that the original turbine performance is maintained in the event of such a replacement, it is necessary that the new turbine rotor blades still have the original radial blade gap width. To that end, the tips of the new rotor blades are correspondingly machined before their assembly. However, since the radial gap width depends not only on the production tolerances of the new rotor blades but also on further influencing factors, the final radial blade gap results only once all the blades have been completely replaced. Influencing factors are in this case for example the profile of the new guide vanes, the profile of the new guide rings, the coating of the new guide vanes and guide rings, the difference in the position of the new blades with respect to that of the replaced old blades, and similar considerations. Calculating the final gap width in advance is correspondingly difficult and can only be done within a very broad tolerance margin.
The machining of the tips of new rotor blades in order to set the radial gap width is normally performed by grinding. This is always performed away from the turbomachine since otherwise there would be the risk of the turbomachine being impaired by the grinding dust produced during the machining process. The blades can thus be machined when mounted on the original rotor disk, for example using a conventional grinding system, although this requires the rotor to be unstacked beforehand, which involves a considerable amount of effort. As an alternative, instead of the original rotor disk, what is termed a dummy rotor disk can be used, whereby the rotor need not be unstacked. However, using such a dummy rotor disk also implies decreased machining accuracy, which can have a negative effect on the performance of the turbomachine. There is further the possibility of machining the rotor blades without a rotor disk, using what is termed a single blade grinding apparatus, which entails further penalties with respect to machining accuracy.